This invention relates generally to overvoltage protection devices for protecting electronic equipment and to methods of making such devices, and more specifically to devices that are commonly referred to as “surge protection” or “transient voltage suppression” devices.
Transient voltage suppression devices have been developed in response to the need to protect the ever-expanding number of electronic devices upon which today's technological society depends from high voltages of a short, or transient duration. Electrical transient voltages can be created by, for example, electrostatic discharge or transients propagated by human contact. Examples of electrical equipment which typically employ transient voltage protection equipment include telecommunications systems, computer systems and control systems.
Some known transient voltage suppression devices include a material having a variable impedance that interconnects, for example, a signal conductor pad and a ground conductor pad formed upon a ceramic substrate or printed circuit board substrate materials. Variable impedance materials, also sometimes referred to as “overstress responsive compositions” are sometimes fabricated as a mixture of conductive and/or semiconductive particles suspended as a matrix within a binding material such as an insulative resin. The signal and ground pads are separated by a small gap on the surface of the substrate, and the variable impedance material is placed into the gap to interconnect the ground and signal conductors. Through-holes or vias extend through the substrate on either end of the device and are plated to provide an electrical path to the signal and ground pads on the substrate. In a surface mount device, one of the plated vias may be connected to a signal conductor or trace of a circuit board, and the other of the plated vias may be connected to a ground conductor trace of a circuit board. The signal and ground pads of the device are therefore connected to signal and ground conductors, respectively, of an electrical system to be protected.
The variable impedance material exhibits a relatively high resistance (sometimes referred to herein as the “off-state”) when the voltage and/or current passing through the signal conductor is within a specified range, and a relatively low impedance (referred to herein as the “on-state”) when the voltage and/or current exceeds a predetermined threshold. In the on-state, the pulse or transient voltage experienced by the signal conductor will be shunted through the device to the ground conductor of the electrical system, and the voltage associated with the pulse will be clamped at a relatively low value for the duration of the pulse. The variable impedance material recovers after the voltage or current pulse has passed and returns to its high impedance state.
While such devices can be effective to protect electronic equipment from transient pulses, they are subject to a number of manufacturing difficulties. For example, the ground and signal pads are typically formed by etching and photolithographic techniques in which layers of conductive material are removed from the substrate, sometimes referred to as a subtractive formation process, to form the ground and signal pads. The gap between the ground and signal pads is typically cut or machined with a laser or other known technique at a separate stage of manufacture from the conductor formation, and controlling the gap formation is difficult and expensive.
Additionally, compounding the variable impedance material involves many processing stages and can also be difficult to consistently produce. Due to the small size of some devices, especially in chip type devices, the variable impedance material can be difficult to apply to the gap, and providing termination structure to connect the device to circuitry can be problematic. Cumulatively, these and other difficulties lead to higher production costs and decreased manufacturing yields of acceptable devices in the fabrication process.
It would be desirable to provide a lower cost and more reliable manufacture of such devices so that transient voltage suppression devices may be produced with increased production yields.